Cisco ASA 5505 Site-tp-Site VPN stops passing traffic

Hi there,
Have two Cisco 5505's - both running version 8.2(1). We have a site to site VPN between them and it works fine. However, the traffic stops passing over the tunnel. The tunnels stay up, but no traffic is passing. I have made sure that the following has been set:
crypto ipsec security-association lifetime seconds 86400
crypto ipsec security-association lifetime kilobytes 2147483647

The lifetime keys runs out before the above data limit is hit as not a huge amount of data passes over the VPN Tunnel
Yesterday for example, I established the tunnels at 8:30am in the morning. By using ping plotter, I could see that the tunnel had dropped at around 5:15am this morning for some reason. This was shown also with the IPsec Session details showing that 12000 secs of the Rekey Left had passed (checked at around 8:30 today, so 12000secs equals roughly 3 1/4 hours)
Not traffic was passing over the tunnel. Under VPN statistics, it was still showing that the tunnel had been up for about 23 odd hours.
I don't want to upgrade to 8.4, as one of the ASAs has a huge config.